In that case my advise would be once you have 2 skills at level 10 and one at level 9 (or close to level 10) to save your game on a seperate save file. Once thats done you can then change your lifetime wish, complete it and then go back to your old game so you can continue to work on completing the Perfect Garden LTW. This will save you from having to start over and is a handy way to complete many LTW's!
As for keeping track of your garden the skill journal will tell you what plants you have grown & their quality. If your struggling to keep up with all your plants try planting less or make sure you purchase the sprinklers and have a
Handy sim upgrade them to auto water.
Also the
Walkthrough for The Perfect Garden Lifetime Wish will help those that haven't read it.